1. Field of the Invention
The present invention relates to a press for powder metallurgy in which a gap between a lower ram and a yoke plate is adequately adjusted to facilitate replacement of a die set and to improve accuracy of a formed product.
2. Description of Invention
In the press for powder metallurgy, it is necessary to replace a die set due to change of a product to be formed. As this type of prior art die set replacing device, there has been known "Die Set Replacement Device for Press for Powder Metallurgy" of Jap.Pat.Publn.No.Sho. 62-33039, "Loader Device for Forming Press" of Jap.Pat.Publn.No.Sho. 62-55960, "Die Set Replacement Device for Press for Powder Metallurgy" of Jap.Pat.Publn.No. Hei 2-37280 and the like. These replacement devices are used for installing or removing a large-sized die set to or out of the main body of the press for powder metallurgy. A small-sized die set is manually replaced.
One example of configuration of the aforesaid prior art die set will be described in reference to FIG. 3. A die set 1 is comprised of component elements illustrated in the FIG. 3 from a yoke plate 2 to an upper punch plate 3. The yoke plate 2 is removably engaged with a lower ram 4 which is installed at a lower part of a main body A of the press for powder metallurgy as it can displace or move vertically. Guide rods 6 are projected upwardly on the yoke plate 2 which is engaged with the lower ram 4. The guide rods 6 pass through a stationary plate 5, and a die plate 7 is secured to the upper ends of the guide rods 6. The stationary plate 5 is secured to the main body A of the press by bolt fastening or a hydraulic clamp through a holder at a predetermined height position. The stationary plate 5 carrys a lower punch 9 which is inserted or fitted into a die 8 disposed at the central part of the die plate 7.
The upper punch plate 3 is removably secured against clamper 11 provided at an upper part of the main body A of the press for powder metallurgy. The clamper 11 is provided to a lower surface of an upper ram 10 which is displaced or moved upward or downward by a driving means not shown. Guide rods 12 passing through the die plate 7 are provided to a lower surface of the upper punch plate 3 so that of the upper punch plate 3 is vertically guided by the guide rods 12. An upper punch 13 inserted or fitted into the die 8 is provided to the lower surface of the upper punch plate 3.
Powder material which is filled in a pressing cavity defined by the die 8 and the lower punch 9 inserted into the die from the bottom side thereof is pressed between the upper and lower punches 9 and 13 so as to form a powder molded product 14.
For installing or removing of the die set 1 to or from the main body A of the press, it is required to align height positions between the upper punch plate 3 and the lower ram 4 for smooth horizontal sliding of the yoke plate 2 in relation to the lower ram 4 with using a replacement device already known as the prior art. It is necessary to provide in general a gap G of about 0.1 to 0.3 mm between the yoke plate 2 and the lower ram for the above operations.
The die set 1 is carried by the main body A of the press via three locations of the upper punch plate 3, the stationary plate 5 and the yoke plate 2. It is known that accuracy of molded product is affected significantly with condition of securing between the yoke plate 2 and the lower ram 4. In order to assure the accuracy of the product, it is desired to fix the yoke plate 2 and the lower ram 4 with no clearence between them so that the gap G is a minimum approximating zero.
Thus, in order to improve the accuracy of a formed product, it is required to arrange the interval of the gap G to a minimum one approximating zero during pressing operation. On the contrary, in order to facilitate replacement of the die set 1, it is desired to arrange the interval of the gap G as large as possible.
A hydraulic clamper or a bolt clamper having a low force might be used as the clamper 11 because no other force than self weight of the upper punch 3 is applied to the clamper 11 during the ascending of the upper ram 10. The clamper part of the stationary plate 5 might be also fixed by the same manner because a force acting in the direction for ascending thereof is not so large too.
During a pressing operation, the lower ram 4 abutting to the lower surface of the yoke plate 2 receives a large load. Meanwhile during an operation that a pressed and molded product is retracted, the yoke plate 2 is concurrently pulled down by the lower ram 4. Due to these facts, clearance between the yoke plate 2 and the lower ram 4 is desirable to be a value as less as possible.
Moreover, in view of that the gap G between the yoke plate 2 and the lower ram 4 provides substantial influence over the accuracy of the molded products 14, said gap G is desirable to be minimum one approximating zero as much as possible. In FIG. 3, a force applied from the upper punch 13 to the lower ram 4 during a pressing operation is indicated with an arrow P1. A force received by the lower ram 4 when the molded product 14 is pulled out of the device is indicated by an arrow P2 in FIG. 4. In a case a large size product is made, the force applied during pulling-out operation sometimes reaches 60% of that applied during pressing operation.
At starting time of molding operation, there is no friction between the die 8 and the lower punch 9 for no powder material lies between them. However if the powder material enters between them accidentally during operation, friction occurs and sometimes becomes a considerably large value. The friction also occurs due to poor accuracy between the guide rods 6 and insertion holes 5a receiving the rods 6.
When the lower ram 4 being lifted up is stopped at a predetermined height in order to fill the powder material into the pressing cavity, an assembly including the yoke plate 2, the guide rods 6 and the die plate 7 is also stopped in a state that it projects upward due to its inertia by a length corresponding to the gap G. Then if the friction is not so large, the assembly descends by its own weight until the lower part of the yoke plate 2 abuts against the upper part of the lower ram 4. Then filling position of the powder material to said pressing cavity might be a certain height. However, if the friction is so large that descending of the assembly which is projected upward by the gap G is prevented with the friction, the filling position of the powder material to said pressing cavity shifts to a position higher than the above certain height by the gap G. Moreover if the friction is exceedingly large, the yoke plate 2 and the lower ram 4 are secured each other with the friction so that no projection of the assembly by the gap G during lifting operation occurs.
Due to the fact that the filling position of the powder material into the pressing cavity is so varied as described above, there may occur disturbances in the filling amount of the powder material so as to cause the accuracy of the pressed and molded product to be remarkably spoiled. For example, if there occurs the disturbance of 0.3 mm in the product having the filling depth of 20 mm, the error in the molded product 14 becomes 0.3/20.times.100=1.5%, resulting in showing an amount which can not be ignored.
Therefore it is an object of the present invention to provide a press of powder metallurgy for solving the problems in the prior art as described above, wherein a large gap is provided between the die set and the lower ram when the die set is to be installed in or removed from the main body of the press, for facilitating displacement of the die set in relation to the main body of the press, while pressing operation is performed with the minimum gap one approximating zero for improving the accuracy of the molded product.